1. Field of the Invention
The present invention relates to an apparatus for analyzing a solid surface by using an X-ray spectroscopy.
2. Description of the Background Art
The field of a solid surface chemistry is a highly important field not only as a field of fundamental research but also as a field which is deeply related to the various industrially useful practical problems such as absorption, catalysis, corrosion, electrode reaction, friction, and electronic characteristics. For example, the catalysis is not only important as it is utilized in the majority of the processes used in the chemical industry, but also as it is considered indispensable in resolving the resource and energy problem and the environmental problem which are becoming the major concerns in recent years.
As a consequence, a large amount of empirical facts has been accumulated in this field over the past years. However, a truly effective means for checking the surface of the matters directly has not been available until recently, so that there has been a long history of estimating the properties of the surface of the matters according to the indirectly obtained information.
In recent years, however, due to the rapid development of various physical and chemical instruments, there is a strong trend to produce new materials and devices with improved functions by appropriately controlling the arrangement, chemical composition, electronic state, vibration state, and depth distribution of the atoms on the solid surface. For this reason, the measurement of the solid surface state is acquiring an increasing significance in recent years.
Presently, the most widely used type of an apparatus for solid surface analysis is a scanning electron microscope (SEM).
In a scanning electron microscope, the electron beam generated from an electron gun is sharply converged by a condenser lens and then irradiated onto a sample surface while performing a scanning operation using deflection coils, and the reflected electrons or secondary electrons emitted from the irradiated sample surface are collected by an electron detector and amplified, such that the amount of the collected electron can be indicated by modulating the luminance on the Braun tube display according to the collected electron beam.
Moreover, by synchronizing the scanning of the electron beam bundles on the sample surface with the scanning on the Braun tube display, it is also possible to obtain the enlarged image of the sample surface in terms of the reflected electron image or the secondary electron image on the Braun tube display.
In a case of observing the sample surface by using such a scanning electron microscope, the amount of the emitted electrons is affected not only by the crystalline orientation but also by the unevenness and the step shaped structure on the surface, so that the contrast of the image reflects all of these factors. Accordingly, it is possible to observe the sample surface in an undestroyed state, without spoiling the sample in terms of the grain size and shape of the crystalline grains, and the shape and distribution of the precipitated materials.
In such a scanning electron microscope, it is possible to achieve a surface resolution of less than 100 .ANG. by narrowing the electron beam, but the emission region for the X-ray fluorescence is rather wide spread. The reason for this is considered to be the following.
Namely, for a sample in which a substrate B has a thin film H formed thereon, in a case of observing the sample by using the electron beam with high energy (20 KeV for example) for the scanning electron microscope, there is a penetration of the electron beam into the sample in the form shown in FIG. 1.
As shown in FIG. 1, the electrons in the scanning electron microscope penetrate through the thin film H and diffuse deeply into the substrate B in a form of a droplet, so that there has been a problem that the X-ray fluorescence is generated not only from the thin film H but also from the substrate B.
Consequently, in the case of carrying out the composition analysis using the X-ray fluorescence excited by the electron beam of the scanning electron microscope, there has conventionally been a problem that the thin film H thinner than the droplet-shaped electron diffusion region cannot be analyzed accurately. In particular, the droplet-shaped electron diffusion region normally has a depth of several .mu.m, so that the analysis of the thin film having the thickness less than 0.1 .mu.m (1000 .ANG.) has been practically impossible.
In addition, the X-ray fluorescence from the thin film H and the X-ray fluorescence from the droplet-shaped region are mixedly observed, so that there has also been a problem that the S/N ratio (ratio of the amount of the X-ray fluorescence from the thin film with respect to the amount of the X-ray fluorescence from the droplet-shaped region) becomes poor.
On the other hand, in 1980, there has been a proposition by Professor Ino of the University of Tokyo and others, for the elementary analysis of the sample surface at a high sensitivity using the excitation of the characteristic X-rays of the atoms constituting the sample surface by the electron beam of the reflection high energy electron diffraction (RHEED) and the detection of the excited characteristic X-rays by an energy dispersive X-ray detector (EDX). This new surface analysis method is known as the total-reflection-angle X-ray spectroscopy (TRAXS).
In this method, the setting of the take-off angle of the characteristic X-rays emitted from the sample in the vicinity of the total reflection angle .theta.c (zero to several degrees) of the X-rays is known to be effective in improving the detection sensitivity for the sample surface elements, so that this method can be effectively utilized for the elementary analysis of the surface. In addition, it is also possible in this method to obtain the information on the atomic configuration on the surface. Furthermore, as this method is applicable to the film of various thickness sizes ranging from a very thin film having layers of only several atoms to a rather thick film, it is also possible in this method to obtain the information on the boundary of the matters.
Now, in the conventional apparatus for carrying out this total reflection angle X-ray spectroscopy method, a detailed structure for the attachment of the energy dispersive X-ray detector for detecting the characteristic X-rays has a configuration shown in FIG. 2.
In this configuration of FIG. 2, a rod shaped probe 112 is pierced through a side wall 111 of a vacuum chamber containing the sample therein, and attached on the side wall 111 by means of a flange 113.
On a tip of this probe 112 placed inside the vacuum chamber, there is provided a window section 107 formed by beryllium (Be) or organic thin film, and the probe 112 contains a semiconductor X-ray detector and FET inside this window section 107.
Also, on the base end side of the probe 112 located outside of the vacuum chamber, there is provided a pulse height analyzer 110. In addition, an L-shaped base end portion of the probe 112 is connected to a heat insulated tank 115 for storing the liquid nitrogen.
The interior of the probe 112 has a vacuum heat insulation structure to which the liquid nitrogen stored in the tank 115 is supplied in order to cool down the semiconductor X-ray detector contained inside the probe 112, so as to reduce the dark current in the semiconductor X-ray detector.
Now, the energy dispersive X-ray detector to be attached to the side wall of the vacuum chamber as shown in FIG. 2 is actually a very expensive device, so that it is preferable for the energy dispersive X-ray detector to be attached detachably such that it can be used in several vacuum chambers, rather than being fixedly attached to a single vacuum chamber.
Moreover, in the case where the energy dispersive X-ray detector is attached to the vacuum chamber directly, at a time of the replacement of the energy dispersive X-ray detector, it becomes necessary to release the vacuum state inside the vacuum chamber, so that the sample cannot be maintained in the vacuum state for an extended period of time, and a considerable amount of time is required for the operation to release the vacuum state.
For this reason, conventionally, the side wall of the vacuum chamber is equipped with a window section formed by beryllium, such that observation can be made by bringing the tip of the probe 112 face to face with the window section provided on the side wall of the vacuum chamber while the probe 112 and the tank 115 are supported outside of the vacuum chamber. With this configuration, the operation of the X-ray detection in several vacuum chambers can be effectively handled by using only one energy dispersive X-ray detector, while avoiding the above described disadvantages related to the release of the vacuum state of the vacuum chamber, and in addition there is an advantage that the handling of the X-ray detection device becomes easier as it can be located in the normal atmosphere.
However, such a conventional apparatus for carrying out this total-reflection-angle X-ray spectroscopy method has been associated with the following drawbacks.
First, there is a case in which it is necessary to form the film on the sample surface directly inside the vacuum chamber in order to carry out the X-ray fluorescence analysis of this film at a time of its formation or immediately after its formation. In such a case, there is a possibility for contaminating the window section 107 provided on a tip of the probe 112 with the film formation materials, and such a contamination of the window section 107 causes the excitation of the characteristic X-rays of the film formation materials during the analysis, so that the error can be introduced into the analysis data.
When such a situation occurs, it becomes necessary to replace the window section 107, but such a replacement of the window section 107 in turn requires the operation of disassembling the probe 112 having the vacuum heat insulation structure, so that considerable amounts of cost, time, and labor are required for the replacement of the window section 107.
Also, in the case where the probe 112 is to be located in the normal atmosphere outside the vacuum chamber, there is an air layer between the tip of the probe 112 and the window section provided on the side wall of the vacuum chamber inevitably, so that a part of the characteristic X-rays are absorbed by this air layer and consequently the detection efficiency is lowered. In particular, the characteristic X-rays having energy below 1.7 KeV are very easily absorbed by the air containing the oxygen, nitrogen, and argon, so that they are hardly detectable in this case. As a result, it is practically impossible in this case to detect the characteristic X-rays of the elements such as C, N, O, F, Na, Mg, and Al which have energy below 1.7 KeV.